Search code examples
sqlregexpostgresqlstring-function

Cannot lowercase backreferenced string in Postgres

I am trying to lowercase a backreferenced string but it wont work. :( I just want to lower the first character after root.

my_string = "<root>Here is July's best food</root>"

Correct one should be:

my_string = "<root>here is July's best food</root>"

Here is my query:

UPDATE my_table
SET my_string = regexp_replace(my_string, '<root>(.)', lower(E'<root>\\1'))
WHERE my_id = 1;
Solution

  • Using split_part() and lower()

     select '<root>'||lower(split_part(split_part('<root>Here is July''s best food</root>',' ',1),'>',2)) || split_part('<root>Here is July''s best food</root>',split_part(split_part('<root>Here is July''s best food</root>',' ',1),'>',2),2)

    Result:

     "<root>here is July's best food</root>"

    UPDATE my_table
SET my_string =  '<root>'||lower(split_part(split_part(my_string,' ',1),'>',2)) || split_part(my_string,split_part(split_part(my_string,' ',1),'>',2),2) 
WHERE my_id = 1;

    following function is optional

    You can make function like below

     create or replace function lower_first_word(txt text,del text) returns text as
$$
 select del||lower(split_part(split_part(txt ,' ',1),'>',2)) || split_part(txt ,split_part(split_part(txt ,' ',1),'>',2),2)
$$
language sql

    and update 

    UPDATE my_table
    SET my_string = lower_first_word('<root>Here is July''s best food</root>','<root>')
    WHERE my_id = 1;

    OR

    Using regex and substring()

    select lower(substring('<root>Here is July''s best food</root>',E'[A-Za-z<>]+'))||substring('<root>Here is July''s best food</root>',E'[^ ]* (.*)')

    and the update statement is

    UPDATE my_table
SET my_string = lower(substring('<root>Here is July''s best food</root>', E '[A-Za-z<>]+')) || substring('<root>Here is July''s best food</root>', E '[^ ]* (.*)')
WHERE my_id = 1;